How to use video preview as video output?
This may not be exactly what you're asking but have you tried rendering
from the Drop down menu at the top left of the 3D window.  It's called "render" strangely enough and gives you access to the "OPenGL Render."

I use this almost always and find it outputs (usually) what I see in the 3D window, or through the camera.  On the few occasions it doesn't, the problem is always a setting or two, (or three, to almost infinity)

A large factor in it's favour is it's rendering speed.